1. Field of the Invention
The present invention relates to apparatus of the kind used to remove excess liquid coating from a moving strip emerging from a coating bath and which operate by directing jet streams of pressurised gas onto the coated surfaces of the strip.
The invention was developed to control the thickness of the zinc or aluminium/zinc alloy coating applied to steel strip in a continuous hot dip galvanising plant, and is described primarily in that context hereinafter. It will be understood, however, that the apparatus of the invention is equally applicable to the control of liquid coatings generally on any moving strip substrate.
2. Description of the Prior Art
In a typical continuous strip hot dip galvanising process, a strip of steel to be coated, after preliminary treatment, passes downwardly into a bath of molten zinc or zinc/aluminium alloy, around a sink roll submerged in the bath, upwardly past at least one deflector roll located just below the surface of the bath, through jet stripping apparatus located closely above the bath, and to and about a turn-around roll located well above the bath.
Traditionally, the jet stripping apparatus has comprised elongate nozzles, one on each side of the strip, extending transversely of the strip and each directing a substantially planar jet stream of gas against the vertical strip. The gas stream acts as a barrier preventing the passage of the outermost layers of the coating material, but allowing a thin, still liquid, inner layer to advance with the strip.
The turn-around roll is the first solid object to contact the coated strip, and it is necessary for the coating to have solidified before contact is made. Having regard to the speed of operation of modern plants, the turn-around roll is, therefore, a considerable distance above the bath, even though strip coolers may be provided to ensure that the coating solidifies before it reaches the roll.
Because of its length, the unsupported strip between the bath and the turn-around roll tends to vibrate. The vibrations cause variations in the distances between the strip and the respective stripping nozzles, and this results in objectionable variations in the coating thickness.
To overcome that disability it has been proposed to provide a gas pressure stabilising pad, sometimes referred to as a floater pad, on each side of the strip above the stripping nozzles. Each such pad comprises a reaction body adjacent to, but spaced from, the strip, and nozzles directing pressurised gas into the space between the strip and the body. Thus, a gas pressure is built up within the space having a value which depends inversely on the leakage from that space, which, in turn, depends on the distance between the body and the strip. The pads are in register, and their net effect is to provide a restoring force whenever the strip wanders from a stable position which, assuming identical pads and gas supplies, is midway between the pads. Floater pads of that kind are described in the complete specification of Australian patent 529545 in the name of Nippon Steel Corporation.
It is also known to combine a floater pad and a gas stripping apparatus into a single unit having two nozzles, both of which contribute gas to the pressurised space between the body of the pad and the strip, and one of which also provides the stripping jet stream. Typically, such a dual-nozzle assembly comprises upper and lower, parallel nozzles, spaced apart by a reaction body. Dual nozzle floater pad/stripping assemblies of that kind are described in the complete specifications of Australian patents Nos. 581081 and 630281 in the name of the present applicant.
If a traditional isolated or stand alone stripping nozzle is used, the stripping jet stream divides, on impingement with the strip, into upwardly flowing and downwardly flowing component streams. It has been found that the upwardly flowing component stream may produce ripples in the surface of the still liquid coating material on the strip above the line of impingement. Thus it has also been proposed to provide a compensating nozzle disposed some distance above each stripping nozzle and liberating a downwardly directed gas stream. That downwardly directed stream annuls or overwhelms the upward flowing component of the stripping jet stream and prevents the formation of the aforesaid ripples. Such a compensating nozzle and its effect are described in the specification of U.S. Pat. No. 3,607,366 in the name of Yawata Iron & Steel Co. Ltd.
In all prior known arrangements as discussed above, the stripping of each side of the strip, that is to say the prevention of the passage of more coating material than that required in the finished product, has been effected by a single gas jet stream, being the stream first met by the strip as it rises from the bath. In dual nozzle stripping/floater pad assemblies the jet stream from the lower nozzle does the stripping and the upper jet stream does not alter the thickness of the liquid coating on which it plays. Likewise, when a compensating nozzle is present, its jet stream plays no part in the reduction of mean coating thickness. It is still the lower jet stream which does the stripping.
The liquid coating dragged up from the bath by the strip has its maximum thickness at the surface of the bath. As it rises further from the bath it is accelerated by viscous drag from the strip, so that its mean upward speed asymptotically approaches that of the strip, which speed is fully attained on solidification, and its mean thickness correspondingly decreases. The stripping jet stream is necessarily positioned where the coating reaching it is still thicker than the required finished coating, and the gas pressure, nozzle outlet width and nominal nozzle spacing from the strip are chosen having regard to the strip speed so as to produce a residual coating layer of the required thickness (typically about 20 micrometers).
While all of the dragged up liquid may be rising with the strip, at least the outer layers move more slowly than the strip and the liquid immediately adjacent the strip. That is to say, relative to the strip and the liquid in direct frictional engagement with the strip, the outer layers of the dragged up liquid, including the liquid coating material which is held back by the stripping jet stream, are moving downwardly in the manner of a falling cascade flowing over a stationary surface. Thus, the coating material below the stripping jet stream takes on the typical appearance of such a cascade, with irregular ripple or wave formations extending generally horizontally across its surface.
It has been found that conventional gas stripping devices are unable to suppress those ripple or wave formations completely, with the result that the finished coating has undesirable surface irregularities and/or the coating has undesirable localised variations in thickness, due to the residual effect of those ripple or wave formations.